1. Field of the Invention
The present invention relates to nonlinear dielectric ceramics for pulse generating capacitors which are used as starters of discharge lamps, pulse generating capacitors in which dielectric ceramics are used as dielectric materials, and high-pressure vapor discharge lamps in which the capacitors are used as starters. More particularly, the invention relates to a nonlinear dielectric ceramic which is suitable for a high-pressure vapor discharge lamp having a starter contained in a bulb.
2. Description of the Related Art
High-pressure vapor discharge lamps, such as a high-pressure sodium lamp, are difficult to start using a commercially-used power-supply voltage and the application of a high pulse voltage is required.
High-pressure vapor discharge lamps, in which starters for generating high pulse voltages are contained in discharge-lamp bulbs and the starters are utilized in combination with ballasts for general high-pressure mercury lamps, have come into wide use. Such a high-pressure vapor discharge lamp basically includes a luminous tube and a capacitor using a nonlinear dielectric ceramic, the luminous tube and the capacitor being connected in parallel, and by combining a solid-state switch (SSS) therewith, a high pulse voltage is generated. The high pulse voltage together with a power-supply voltage is applied to the luminous tube to start the discharge lamp.
As a means for stably generating such a high pulse voltage, a pulse generating capacitor including a nonlinear dielectric ceramic composed of a barium titanate-based compound as a dielectric material has been used.
The pulse generating capacitor has a D-E hysteresis curve in which electric displacement (D) changes steeply in relation to voltage (E), as shown in FIG. 1. If a voltage that is larger than the coercive electric field of the capacitor is applied, the electric charge is abruptly saturated in the vicinity of a polarization inversion voltage. A change in the electric current at this stage also causes a change in ballast, and a high pulse voltage corresponding to xe2x88x92Lxc2x7di/dt can be generated due to the inductance of the ballast.
The pulse generating capacitor used for a high-pressure vapor discharge lamp such as a high-pressure sodium lamp must have a steep slope of the D-E hysteresis curve, which must be stable over a wide temperature range. Pulse generating capacitors which meet the above requirements are disclosed in Japanese Unexamined Patent Publication Nos. 63-221504, 63-221505, 1-136323, 1-136324, etc.
The bulb of a high-pressure vapor discharge lamp, such as a high-pressure sodium lamp, is usually maintained at a high vacuum of approximately 1xc3x9710xe2x88x925 torr, and is exposed in a high-temperature high vacuum of 300xc2x0 C./1xc3x9710xe2x88x925 torr. A barium getter for adsorbing oxygen generated during lighting is also disposed in the bulb of the discharge lamp so that the degree of vacuum in the bulb is maintained. However, if the discharge lamp continues to be lit, a reducing atmosphere is produced in the bulb due to hydrogen adsorbed by members such as a luminous tube, the metallic support for the luminous tube, the glass constituting the bulb, and hydrogen generated by the decomposition of adsorbed water.
Therefore, if the pulse generating capacitors disclosed in Japanese Unexamined Patent Publication Nos. 63-221504, 63-221505, 1-136323, 1-136324, etc. are used in bulbs for a long period of time, the dielectric ceramics are reduced and the insulation resistance is decreased, resulting in a low or no pulse voltage being generated, and hence the discharge lamp is not lit.
In order to cope with the above problems, as disclosed in Japanese Unexamined Patent Publication No. 60-52006, the pulse generating capacitor except for a current-carrying section is entirely coated with inorganic glass, or as disclosed in Japanese Unexamined Patent Publication No. 4-34832, a getter for adsorbing hydrogen is disposed in the bulb. However, deterioration is not fully suppressed by the above measures, and the structures of the pulse generating capacitors and discharge lamps may become complex, resulting in an increase in cost. Additionally, if the pulse generating capacitor is entirely coated with inorganic glass as disclosed in Japanese Unexamined Patent Publication No. 60-52006, the D-E hysteresis characteristics of the dielectric ceramic is degraded by the glass, and it may become difficult to obtain a high pulse voltage. Furthermore, production problems may be caused. For example, inconsistencies in the characteristics in the different dielectric ceramic lots may be increased, and warpage may occur in the ceramic when it is fired.
Accordingly, it is a main object of the present invention to provide a nonlinear dielectric ceramic which is suitable as a dielectric material for obtaining inexpensive pulse generating capacitors, in which the characteristics are not degraded even if exposed in high-temperature high vacuum during lighting and in a reducing atmosphere during use, a high pulse voltage can be generated over a wide temperature range, and inconsistencies in characteristics are decreased, thus enabling stable characteristics in terms of production.
It is another object of the present invention to provide a pulse generating capacitor in which a nonlinear dielectric ceramic is used as a dielectric material.
It is another object of the present invention to provide a high-pressure vapor discharge lamp circuit or a high pressure vapor discharge lamp which uses the pulse generating capacitor as a starter.
In accordance with the present invention, a nonlinear dielectric ceramic having the D-E hysteresis characteristics contains a barium titanate-based compound as a principal constituent and a nonreducing oxide glass as a secondary constituent, and thus the nonlinear dielectric ceramic has reduction resistance.
The content of the nonreducing oxide glass is preferably set at about 0.8 parts by weight or less relative to 100 parts by weight of the barium titanate-based compound.
Preferably, the nonreducing oxide glass contains at least one of Si and Li.
Preferably, the nonreducing oxide glass is one of a first nonreducing oxide glass and a second nonreducing oxide glass, the first nonreducing oxide glass being represented by the formula Li2Oxe2x80x94(Si, Ti)O2xe2x80x94MO (where MO is at least one of Al2O3 and ZrO2), and the second nonreducing oxide glass being represented by the formula SiO2xe2x80x94TiO2xe2x80x94XO (where XO is at least one oxide selected from the group consisting of BaO, CaO, SrO, MgO, ZnO and MnO).
Preferably, the first nonreducing oxide glass is represented by the formula xLi2O-y(SiwTi1xe2x88x92w)O2-zMO (where MO is at least one of Al2O3 and ZrO2, x, y, and z refer to mole %, and subscript w satisfies the relationship 0.30xe2x89xa6wxe2x89xa61.0). In the ternary composition diagram shown in FIG. 2, the ratio (x, y, z) lies within a polygon, including the sides of the polygon, obtained by linking point A (20, 80, 0), point B (10, 80, 10), point C (10, 70, 20), point D (35, 45, 20), point E (45, 45, 10) and point F (45, 55, 0), (wherein w satisfies the relationship 0.30xe2x89xa6w less than 1.0 if the ratio lies on the line A-F).
Preferably, the second nonreducing oxide glass is represented by the formula xSiO2-yTiO2-zXO (where XO is at least one oxide selected from the group consisting of BaO, CaO, SrO, MgO, ZnO and MnO and x, y, and z refer to mole %). In the ternary composition diagram shown in FIG. 3, the ratio (x, y, z) lies within a polygon, including the sides of the polygon, obtained by linking point A (85, 1, 14), point B (35, 51, 14), point C (30, 20, 50) and point D (39, 1, 60).
Preferably, the second nonreducing oxide glass contains at least one of Al2O3 and ZrO2 in an amount of about 15 parts by weight or less in total (where the content of Zro2 is about 5 parts by weight or less) relative to 100 parts by weight of the SiO2xe2x80x94TiO2xe2x80x94XO-based nonreducing oxide glass.
Preferably, the second nonreducing oxide glass contains at least one of Li2O and B2O3 in an amount of about 20 parts by weight or less in total relative to 100 parts by weight of the SiO2xe2x80x94TiO2xe2x80x94XO-based nonreducing oxide glass.
Preferably, the barium titanate-based compound is represented by the formula (Ba1xe2x88x92xxe2x88x92yxe2x88x92zSrxCayMgzO)mxc2x7(Ti1xe2x88x92oxe2x88x92pZroHfp)O2, wherein subscripts x, y, z, m, o, and p satisfy the relationships 0xe2x89xa6xxe2x89xa60.05, 0xe2x89xa6yxe2x89xa60.02, 0xe2x89xa6zxe2x89xa60.005, 0.995xe2x89xa6mxe2x89xa61.02 and 0.035xe2x89xa6o+pxe2x89xa60.12 (where 0xe2x89xa6oxe2x89xa60.12 and 0xe2x89xa6pxe2x89xa60.12).
Preferably, the nonlinear dielectric ceramic contains an oxide of at least one element selected from the group consisting of La, Ce, Nd, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb and Y in an amount of about 0.5 mole or less relative to 100 mole of the barium titanate-based compound.
Preferably, the nonlinear dielectric ceramic contains an oxide of at least one element selected from the group consisting of Mn, Ni, and Co in an amount of about 0.5 mole or less relative to 100 mole of the barium titanate-based compound.
Preferably, the nonlinear dielectric ceramic contains an oxide of at least one element selected from the group consisting of La, Ce, Nd, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb and Y and an oxide of at least one element selected from the group consisting of Mn, Ni, and Co in an amount of about 1.0 parts by weight or less in total relative to 100 parts by weight of the barium titanate-based compound.
A pulse generating capacitor in accordance with the present invention is used in a bulb of a high-pressure vapor discharge lamp, and the capacitor includes a dielectric material and electrodes formed on the dielectric material. The dielectric material is composed of the nonlinear dielectric ceramic.
A high-pressure vapor discharge lamp circuit in accordance with the present invention includes a series circuit composed of the pulse generating capacitor and a switch and a luminous tube, the series circuit being electrically connected to the luminous tube in parallel.
A high-pressure vapor discharge lamp in accordance with the present invention includes the pulse generating capacitor and a luminous tube which are electrically connected to each other in parallel and enclosed in a bulb.